Quasi‐Lipoxygenase Activity of Haemoglobin

Haemoglobin catalyses at low concentrations (0.01–1 μM) a quasi‐lipoxygenase reaction with remarkably high substrate specificity. Formation of lipohydroperoxides was demonstrated only with free 9‐ cis ,12‐ cis ‐octadecadienoic acid (Linoleic acid), its glycerol monoester, or 9‐ cis ,12‐ cis ‐octadecadienol (Linoleoyl alcohol). All‐ cis ,‐9,12, 15‐octadecatrienoic acid (α‐linolenic acid), all‐ cis ,‐5,8,11,14‐eicosatetraenoic acid (arachidonic acid) and all‐ cis ,‐9,12, 15‐ eicosatetraenoic acid (dihomo‐ν‐linolenic acid), phospholipids and biological membranes were not attacked. Saturated and unsaturated free fatty acids other than linoleic acid were strong inhibitors of the haemoglobin‐catalyzed oxygenation of linoleate. The reaction was also inhibited by cyanide, carbon monoxide, phenolic antioxidants and the lipoxygenase inhibitor salicylhydroxamic acid, as well as by some anti‐inflammatory drugs (prednisolone, dexamethasone, acetylsalicylic acid); indomethacin did not inhibit. Protein inhibitors of liproxygenase reactions were found in lung homogenates of various species. The molecular activity of the haemoglobin‐catalyzed oxygenation of linoleate was comparable with those of true lipoxygenases and was strongly suppressed by denaturation (heating, urea, pronase treatment, acid splitting). The apparent S 0.5 , the activation energy and the pH‐optimum did not differ from those of most of the true lipoxygenases. The reaction product hydroperoxylinoleic acid but not hydrogen peroxide was an activator of the haemoglobin‐catalyzed oxygenation of linoleate. Hydroxylinoleic acid was a competitive inhibitor of the activation. At haemoglobin concentrations higher than 1 μM the quasi‐lipoxygenase activity was completely suppressed presumably owing to the formation of inhibitory concentrations of hydroxylinoleate via the lipohydroperoxidase [21] activity of haemoglobin. The quasi‐lipoxygenase reaction of haemoglobin exhibits a suicidal behaviour, caused by destruction of haem groups. The properties of the free haemin‐catalyzed reaction were similar to those of haemoglobin. Among other haemoproteins tested myoglobin and cytochrome P‐450 LM show comparable activities. The mechanism of the haemoglobin‐catalyzed oxygenation of linoleate seems to involve a change of valency of the haem iron.